Second part of hashed IP_reassembly changes:
When under pressure for mbufs or we have too many fragments in the IP
reassembly queue, drop half of all fragments. This multiplicative-drop
strategy ensures we return to a healthy state, even under borderline
denial-of-service from extremely lossy NFS-over-UDP peers.
The multiplicative-drop phase currently drops 50% of fragments, but
has pre-placed support for implementing drop-fractions other than 50%
The threshhold for the `drop-half' phase is the new variable,
ip_maxfrags which is calculated as nmbclusters/4.
ip_input.c now keeps ip_nmbclusters, a cached copy of nmbclusters.
Before using limits derived from nmbclusters, we check if nmbclusters
and ip_nmclusters are equal. If not, we recompute Ip parameters
derived from nmbclusters. Based on a suggestion by Jason Thorpe.
ip_maxfrags is currently auto-recalcuated.
The counters ip_nfrags and ip_nfragpacketsr are now declared static
and uninitialized (bss), to discourage tampering with them.